Films are often applied to transparent substrates, such as lenses, goggles, helmets, and visors, in order to improve one or more physical properties of the transparent substrates. Multi-layer film compositions may be used in applications that require a combination of desired properties. For example, one layer may provide improved structure or durability while another layer may inhibit fogging. The combination of such layers results in compositions having improved strength/durability and anti-fogging characteristics.
Many fog resistant films are known. For example, polyurethane or silane coatings may be applied directly to substrates with no separation therebetween, with or without adhesive, to impart anti-fog properties, e.g., to prevent the formation of water droplets thereof. In use, however, such layers may separate from one another, creating performance and/or durability problems.
Other conventional anti-fog films utilize a one-piece construction. These fog resistant films may be formed by treating a cellulose acetate-containing substrate to impart anti-fog properties. The cellulose acetate substrate is typically formed by combining cellulose acetate and one or more specific plasticizers, e.g., phthalates such as diethyl phthalate.
The use of some plasticizers, however, often causes compatibility problems with the other layers of the multi-layer film composition and/or with structural features such as pins or connectors that extend through the layers in order to secure the orientation thereof relative to a housing, such as a motorcycle helmet. For example, where diethyl phthalate is the plasticizer and the pins or connectors (or potentially the protective layer) come into contact with the diethyl phthalate in the anti-fog layer, the diethyl phthalate may degrade the material of the pins or connectors that extend through the multi-layer composition. In some cases, the extension of the pins through the anti-fog layer causes the pins to contact the middle portion of the anti-fog film (where much of the plasticizer may be present). This contact results in plasticizer migration to the pins. In some cases, the plasticizer may even migrate into the other layers. This migration undesirably can lead to degradation causing brittleness and cracking as well as hazing or a lack of clarity. In some cases, other plasticizers, e.g., glycerol tribenzoate, benzyl benzoate, (acetyl) triethyl citrate, may contribute to significant haze problems, which detrimentally affect the clarity of the multi-layer film composition.
In view of the foregoing drawbacks, the need exists for a multi-layer film composition having protective and anti-fog properties, in which degradation, brittleness, and haziness resulting from plasticizer migration are eliminated or minimized.